1. Field of the Invention
The present invention relates generally to wastewater treatment systems and methods, and, more particularly, to such systems and methods using bioremediation techniques.
2. Related Art
Engineered wetlands for wastewater treatment are known to have three basic hydraulic configurations: surface flow (SF), subsurface horizontal flow (SSHF), and vertical flow (VF). The first two are believed the most common, and are known to have significant design shortcomings. Even though an early wastewater treatment wetland design utilized vertical flow, design criteria are still considered experimental for vertical flow wetlands. Surface-loaded, vertical-flow wetlands are believed advantageous because surface loading forces flow through the root zone.
The basic hydraulic flow path for VF wetlands is for wastewater to be introduced at the wetland surface, pass through media and plant roots, then to flow out of the wetland via an underdrain system. Vertical flow wetlands are often designed to have a period of filling followed by a period of draining. When filled by wastewater, bacterial metabolism within the media depletes dissolved oxygen, producing anoxic or anaerobic conditions. As water drains, air is drawn down into wetland media, which is important to permit aeration of wetland media. Drain and fill cycles with a period of approximately a day or less are termed tidal flow. Previously known tidal flow systems are believed to have poor denitrification performance, with the exception of a reciprocating tidal flow system as taught by Behrends (U.S. Pat. No. 5,863,433).
Subsurface horizontal flow wetlands tend to provide better BOD5 and TSS treatment than SF wetlands. Despite this advantage, both BOD5 and TSS effluent values from SSHF wetland cannot reliably be expected to meet tertiary treatment standards. Nitrification in SSHF wetlands is notoriously poor.
An advantage of SSHF wetland design is that with no exposed water surface there is no place for disease vectors to breed. In practice this advantage is often not realized because of surfacing and ponding of wastewater resulting from clogging of wetland media. Surfacing and ponding of wastewater in SSHF wetlands is inherent to most designs. Interstices in gravel media eventually fill with organic and inorganic substances carried in or generated from the wetland influent. Channeling then occurs within the wetland media, degrading treatment. Horizontal flow path velocities are insufficient to carry inorganic fines and recalcitrant organic materials through the media to the wetland outlet. Typically, the inlet of the wetland will dog, forcing wastewater to the surface. Although wastewater will eventually submerge again into the downstream media, some ponding is unavoidable in this situation. Vectors can breed in these free wastewater surfaces.
With regard to the role of plants in SSHF wetlands, water flows through the gravel media in which plants are rooted. Contact of wastewater with plant roots has previously been thought to play a significant role in treatment; however, there is growing evidence that this is not the case. Results from studies comparing vegetated and unvegetated subsurface flow wetland treatment systems indicate that plants do not significantly impact treatment, even though there is strong evidence that the presence of roots in SSHF wetlands significantly affects the composition of microbial populations.
Findings of little or no contribution to treatment from plant roots in SSHF wetlands probably arise from the relationship between roots and media, and the growth characteristics of roots. The treatment effect of roots is likely to be poorly distinguished from that of media if the media surface area is very large compared with that of plant roots. Moreover, in subsurface horizontal flow wetlands roots tend to grow little below the permanently wetted media surface, creating only a shallow zone of root penetration. The greater hydraulic resistance created by the plant roots reduces wastewater flow in this zone. A dead zone frequently results due to the deposition of organic material and the lack of circulation and re-aeration in this zone. Obviously, the potential treatment role of roots cannot be determined if there is minimal root contact with wastewater.
Studies by Tanner et al. do provide convincing evidence that plants can play a significant treat role in flood and drain wetland cells. More research is required to further explore the role of plants in treatment wetlands. The emerging picture thus appears to be that the role of plants is sensitive to wetland hydraulic regime. It is likely that media selection also affects the apparent treatment role of plants.
Even though an early wastewater treatment wetland design utilized vertical flow, design criteria are still considered experimental for vertical flow wetlands. Surface-loaded, vertical-flow wetlands are believed advantageous because surface loading forces flow through the root zone.
The basic hydraulic flow path for VF wetlands is for wastewater to be introduced at the wetland surface, pass through media and plant roots, then to flow out of the wetland via an underdrain system. Vertical flow wetlands are often designed to have a period of filling followed by a period of draining. When filled by wastewater, bacterial metabolism within the media depletes dissolved oxygen, producing anoxic or anaerobic conditions. As water drains, air is drawn down into wetland media, which is important to permit aeration of wetland media. Drain and fill cycles with a period of approximately a day or less are termed tidal flow. Previously known tidal flow systems are believed to have poor denitrification performance, with the exception of a reciprocating tidal flow system as taught by Behrends (U.S. Pat. No. 5,863,433).
Most work with VF wetlands has been done in Europe, employing fine, sharp sand at the surface, underlain with coarser media. Plants root in the fine sand. The low hydraulic conductivity of the fine sand forces a temporary free water surface. Slow percolation through the saturated sand layer is thought to aid treatment. After completely draining, the previously flooded wetland cell is allowed to rest for a period, usually a few days, to permit reaeration of the sand layer. Without reaeration the sand in the interstices would eventually dog with accumulated wastewater constituents and biomass growing on wastewater nutrients.
European VF wetland designs appear to provide superior BOD5 removal, nitrification, and total nitrogen removal than SF and SSHF wetlands, but removal of TSS may be better in SSHF wetlands. Some treatment wetlands are designed in combination, employing a VF wetland for nutrient removal, then followed by an SSHF wetland for TSS removal. Vertically loaded wetlands in series, followed by SSHF wetlands, have been investigated as well.